1. Field
The disclosed embodiments generally relate to battery packs comprised of multiple battery banks which are coupled together in series. More specifically, the disclosed embodiments relate to a method and an apparatus for balancing voltages between battery banks within a battery pack.
2. Related Art
Battery performance is critical to the effective operation of portable computing devices, such as laptop computers. To provide higher supply voltages, battery banks inside portable computing devices are typically stacked in series inside a battery pack. This arrangement provides power efficiently because conduction losses are lower in such a series arrangement. (Note that a battery bank can include one or more battery cells which are electrically connected together in parallel.)
However, if the battery banks that comprise the battery pack are not precisely matched in capacity, the battery pack can suffer from an imbalance condition. Such bank imbalance conditions can exist in new battery packs due to manufacturing variations between the banks, or they also can arise over the life of a battery pack as bank capacities degrade at different rates over time. An imbalanced battery pack has reduced capacity because the bank with the highest state-of-charge will cause the charging process to terminate, which means that banks with a lower state-of-charge never get fully charged. Additionally, when the battery pack is discharged, the bank with the least charge can cause the discharging process to stop, even though charge may remain in other banks
A number of mechanisms are presently used to deal with imbalance conditions in battery banks “Passive balancers” operate by switching resistances in parallel with selected battery banks during the charging process. These resistances act to divert current around the selected banks during the charging process, which causes the selected banks to charge more slowly, which facilitates equalizing the voltages across the banks during the charging process. Although passive balancers can equalize bank voltages during the charging process, they do not alleviate imbalance problems that arise during the discharging process.
In contrast to passive balancers, “active balancers” are inductor based and can operate at any time, for example while the battery pack is charging, discharging or at rest. Active balancers operate by selectively coupling inductors to battery banks to move current between the battery banks Unfortunately, such active balancers can create safety problems. For example, if the switching process is not controlled carefully or if there is a failure in a switch, it is possible to push too much current into a battery bank, which can damage the battery bank.
Hence, what is needed is a method and an apparatus for addressing capacity imbalance problems between battery banks without the drawbacks of existing passive balancers or active balancers.